What are the primary causes of hypernatremia in elderly individuals or those with underlying neurological disorders?

Causes of Hypernatremia in Elderly and Neurologically Impaired Patients

Primary Mechanisms of Hypernatremia

Hypernatremia (serum sodium >145 mmol/L) results from either net water loss or sodium gain, with water deficit being the predominant mechanism in elderly and neurologically impaired populations. 1, 2

Water Loss (Most Common)

Inadequate water intake is the leading cause in elderly and neurologically impaired patients:

• Impaired thirst mechanism - Age-related decline in osmoreceptor sensitivity reduces the drive to drink water 2, 3
• Inability to access water - Patients with stroke, dementia, or physical disabilities cannot independently obtain fluids 2, 4
• Impaired consciousness - Delirium, encephalopathy, or sedation prevents recognition of thirst 5, 4

Renal water losses:

• Diabetes insipidus (central) - Traumatic brain injury, stroke, neurosurgery, or CNS infections damage the hypothalamus or posterior pituitary, eliminating ADH secretion 1, 2
• Diabetes insipidus (nephrogenic) - Lithium therapy, hypokalemia, hypercalcemia, or chronic kidney disease impair renal response to ADH 1, 2
• Osmotic diuresis - Hyperglycemia (>180 mg/dL), mannitol administration, or high-protein tube feeds cause obligate water loss 1, 4

Extrarenal water losses:

• Gastrointestinal losses - Diarrhea, particularly osmotic diarrhea from tube feeds or lactulose, causes hypotonic fluid loss 1, 2
• Insensible losses - Fever, tachypnea, or mechanical ventilation without adequate humidification increases water loss 5, 4
• Burns or open wounds - Extensive skin damage causes massive evaporative water loss 5

Sodium Gain (Less Common)

Iatrogenic sodium administration:

• Hypertonic saline infusions - 3% NaCl for cerebral edema management or rapid hyponatremia correction can overshoot 1, 2
• Sodium bicarbonate - Aggressive administration during cardiac arrest or severe metabolic acidosis 1
• Tube feeding formulas - High-sodium enteral nutrition without adequate free water flushes 4

Endocrine causes:

• Primary hyperaldosteronism - Chronic aldosterone excess causes sodium retention, though typically mild hypernatremia 1
• Cushing syndrome - Cortisol excess promotes sodium retention 1

Special Considerations in High-Risk Populations

Elderly patients have multiple compounding risk factors:

• Reduced total body water (from 60% to 50% of body weight) amplifies the impact of water deficits 2, 3
• Decreased renal concentrating ability (maximum urine osmolality declines from 1200 to 800 mOsm/kg) impairs water conservation 2, 4
• Polypharmacy with diuretics, laxatives, or anticholinergics exacerbates losses 2

Neurological disorders create unique vulnerabilities:

• Stroke patients - Dysphagia prevents adequate oral intake; central DI from hypothalamic involvement 1, 2
• Dementia patients - Forget to drink; cannot communicate thirst; may refuse fluids 2, 3
• Parkinson's disease - Impaired swallowing mechanics and autonomic dysfunction 2

Critical Pitfalls to Recognize

Hypernatremia in pediatric populations on parenteral nutrition often results from incorrect replacement of transepidermal water loss (TEWL), inadequate water intake, or excessive sodium intake during the transition phase, particularly in very low birth weight infants 6

Rapid correction of hypernatremia (>10-15 mmol/L per 24 hours) can induce cerebral edema, seizures, and neurological injury - the brain adapts to hyperosmolality by generating idiogenic osmoles, which draw water intracellularly when sodium drops too quickly 6

Vulnerable water homeostasis in elderly patients creates a tendency toward both hypo- and hypervolemia, with increased extracellular water and decreased intracellular water making them prone to rapid shifts 6